newenvironment{myenv}{
  letmycaptionmyns@mycaption

  begin{figure}[!h]
  caption{???whatgoeshere???}
  end{figure}
}{}

begin{myenv}
  mycaption{Hello world}
end{myenv}

newenvironment{myenv}{
  letmycaptionmyns@mycaption

  begin{figure}[!h]
  if @mycaption is filled out
  caption{@mycaption}
  fi
  end{figure}
}{}